The present invention relates generally to methods and systems for inspecting defects on wafers, masks, and reticles. More particularly, the present invention relates to optical inspection systems and techniques.
In a conventional optical inspection system, defects are detected by subtracting a reference image from a test image to produce a difference image. The test image is an optical image of an area, for example, on a photomask or a wafer. The reference image may be an optical image of a similar area on an identical die or on the same die or rendered from a design database. The grayscale residues, i.e., portions of the difference image having a value other than zero, may represent defects in the inspected sample.
Conventional optical metrology techniques use intensity based or scattering based systems. With intensity based systems it is difficult to detect dark defects sitting in dark structures or to differentiate from other defects with similar contrast. High Aspect ratio Inspection (HARI) refers to the inspection of High Aspect Ratio (HAR) structures. HARI provides difficult wafer inspection challenges to chipmakers, especially as the integrated circuits become smaller. Because the background is darker, slight perturbations may not easily be discriminated against the background. Furthermore, dark structures provide relatively low signal levels.
For example, and as illustrated in FIG. 1, the intensity 102 of the reflected signal from a contact hole 104 having an aspect ratio of 3:1 is very low in the vicinity of the hole 104. The normalized intensity 106 for the surface areas 108 outside the contact hole 104 is shown in the illustration to be considerably larger. This plot illustrates the reflected intensity profile under normal coherent illumination. Thus, defects residing on the trench floor, i.e., the bottom of a HAR structure, would be difficult to discriminate against such a low intensity (dark) background as illustrated by intensity level 102. Moreover, other background noise, such as from misalignment between images for example, may mask slight differences in intensity attributable to the defect.
Laser scattering metrology techniques also rely on the interaction between the illumination and the defect structure, and therefore have similar limitations in identifying defects in HAR structures. Device miniaturization trends are expected to exacerbate this problem. As smaller geometries are used, defects in the more predominant HAR structures will be difficult to detect with current tools. E-beam inspection tools are currently available but are unsuitable for inspection inline. E-beam techniques do not provide a very high throughput.
Accordingly, what is needed is an inspection technique that is capable of reliably identifying defects in high aspect ratio structures and other subtle defects and which provides reasonable throughput so as to integrate the new inspection tool into the semiconductor production process.